Which is the most complex sport in the world?

Test Cricket

This is indeed a very subjective and controversial question. You cannot compare sportsman from different fields, who is better Usain Bolt or Michael Phelps? You even can't compare sportsman of different eras in the same sport, Sachin Tendulkar and Don Bradman, Maradona and Pele, Federer and Bjorg. To compare sports would be even more subjective, but this question has been troubling me being an avid follower of almost all major sports.
 
The question is not which is the most popular, football without doubt, or which is the most entertaining, I personally find rugby as the most entertaining one, but which is the most complex sport, it is from the point of people playing the sport and not the people watching. The complex the sport is the more factors a player needs to consider when they are playing it. A highly complex sport is thus very difficult to master.
 
I have divided the whole argument into comparisons on some specific, easily objectified points, namely

·  Strategy
·  Teamwork and individual brilliance
·  Game-play 
·  Physical fitness
·  Destiny in your own hands - Possibility of comeback
·  Ease of substitution
·  Variations in game-play
·  Effect of external factors

All sports can be divided into either Team or individual. Both genres have their particular attributes on the above mentioned points. I am focusing on only team sports since there is an added dimension of team work and compatibility which makes these sports more complex.
 
Among the team sports, I am focussing on only the most widely played namely: Football, Basketball, Baseball, Cricket, American football, Hockey and Rugby, because there might be a very complex sport which only a 100 people play. 

Strategy 

Strategy implies, how much work needs to be done before the game actually starts, it is different from playing culture, for e.g. Barcelona playing “tiki taka” is their culture not their strategy, while India playing 3 quick bowlers at Perth is strategy. It is highly dependent on your opponent. 
 
In case of team sports, strategy is very important. American football is the most strategic sport, since every play is planned over and over again. It’s like a set play in football but the whole game has only set plays, thus a high level of planning is required. Another important factor is how much you study a particular opponent. Teams develop strategies to contain players like Messi, Ronaldo; they study their game and make plans accordingly. In most ball sports i.e. football, hockey, basketball, this is done for only very special players. Since the game is very dynamic it cannot be done for every player. Baseball and cricket on the other hand, there is a strategy for every batsman/hitter and bowler/pitcher. Further is cricket the field positions have still further possibilities with respect to the batsman, making it a very strategic sport.

Teamwork and individual brilliance

In all team sports, team work plays a very integral part, but how important is an individual to a team. How well can individual brilliance affect the game? For a majority of team sports football, hockey, and rugby its 11 against 11 or 15 against 15 simultaneously, thus the individual won't be able to affect the game to a very great extent. The type of game is also very important for understanding this point, in case of basketball where it is still 5 versus 5; an individual is still a bigger contributor then the above mentioned sports, primarily because scoring a basket is a highly individual event as compared to scoring a goal. In games like cricket and baseball, it is effectively 1 versus 1 and not 11 versus 11; this implies that an individual is much more powerful in these sports. The dependencies on other players of their own team are very less. 
 
Thus the degree of team work and individual impact are both inversely related.

Game-play

Game-play implies what is the framework on which the game is played i.e. football is played over 90 minutes, basketball over 48 minutes etc. The pace at which the game is played is also very important when we are considering impact of game-play; can a team take stock of the situation while playing the game? Can a team change strategy in between the game? 
 
In case of football, there are no stops for 45 minutes straight, thus in most cases the strategy of the team is changed during the half. This is same for sports like hockey and rugby. For basketball, American football, there can be timeouts where the team rethinks their game plan. In baseballs case the end of every inning allows players to regroup. The most however is in case of cricket, where after every over, in tight situations even after every ball a new strategy can be worked upon. This ability to change your game plan frequently increases the sports complexity greatly. 
 
The role of the captain also becomes very important in sports where decisions are needed to be made at very frequent intervals. Thus in a sport like cricket, being a captain has a huge additional responsibility, unlike other sports.

Physical fitness

The fast pace of basketball requires great physical fitness, but then players can be substituted very easily. American football might be a very physical sport but apart from a few positions such as the line backer, you don't require a huge amount of fitness as compared to say footballers of hockey players who play for 45 and 35 minutes straight. The most physically demanding would be Rugby since the nature of the game implies that you have to be strong and the gameplay requires playing for 40 minutes straight. Baseball and cricket do require certain levels of fitness but cannot be compared to the other sports.

Destiny in your own hands - Possibility of comeback

Almost all team sports are time-bound, thus in effect the team has to achieve something within certain time constraints. Thus the team never truly has destiny in their hands. If a score of a football match is 2-0 with 3 minutes to go, the main opponent of the trailing team becomes the clock, they might have the calibre to score 3 goals, they just don't have the time.
 
Baseball however allows team to play till they get 3 outs. So even if the team is 15-0 down in the bottom of the 9th, it still can win on its own merit, not depending on anyone else, time is not a factor, the last inning may drag out for a while. The penalty shootout can also be considered in a similar sense, but since they are not a part of the standard game-play, it is not the same. Taking time out of the equation makes the sport more unpredictable and thus more complex.  It must be noted that in most individual sports, time is never in question.

In Rugby if the trailing team has the ball and has a chance of winning the game from the last play, time becomes inconsequential. Thus, although you cannot expect dramatic comebacks, their destiny is still in their hands. Test cricket, which is played out over 5 days, to some extent is able to take time out of the equation, but it is still bound by it.

Ease of substitution 

Sports like hockey, basketball allow rolling substitutions, football has fixed number, in baseball and american football, you can change your entire side at one go, while cricket does not allow any changes. If the player has a bad day he can be changed, the team does not have to face the effect of an individual’s failure to perform. The increase in amount of players that can a play a particular match increases the complexity in terms of strategy against particular players but makes the process of team selection very difficult and complex. If a bowler is not performing in a test match, you have to live with it for the whole 5 days, while a pitcher can be substituted after 2 bad pitches. The importance of team selection and the risk of player non-performance is very high in sports where substitution is limited or not allowed.

Variations in game-play

Almost all sports are standardized. Their variations have themselves become stand-alone sports themselves for e.g. futsal and beach football are variations of football, Rugby 7's is a variant of rugby etc. Players don't play in all the variations simultaneously. 

However Cricket is a sport that has 3 major variations, Test, T20 and One day. These variations have the same basic rules, but vary with respect to time and other specifics. Almost all players play all the 3 variants in the professional career. Having 3 variations which require a slightly different skill set greatly increases the complexity of a sport.

Effect of external factors

Basketball, air hockey are completely independent of external factors, they are played indoor in a climate controlled environment. They are highly standardized with respect to this point. In outdoor sports like football, rugby, climate is a variant. A rainy day implies that the players have to adjust their game slightly; this too however is not a big change. In case of hockey, the surface is very important, the change from real grass to synthetic turf deprived India of a few Olympic medals at-least. However this now has been also standardized. The types of sticks used in hockey or the type of bat in baseball can also be considered as external variants.

However Cricket is the sport which has the highest effect of external factors. The main difference with other sports is the main element of the sport, the ball, varies to a great degree. In all other sports, the ball, puck is very standard. In cricket’s case, changes to the ball like its colour, shine, age affect the game-play. Another important variant is the pitch; a lot of team selection decision is taken keeping in mind the pitch of the venue, something which is standardized in other sports.

At the end, every sport has a feature that makes it stand out from the rest. In terms of complexity, cricket mainly due to the variations in game-play and the effect of external factors on the game make it the most complex in the world. 
 
To be frank this is a very subjective discussion and it is open to debate.